Narrative:

We lost both pressure controllers at cruise. [We] switched to manual pressurization per QRH. No emergency was declared due to manual pressurization performing adequately. Briefed first officer that he would be the 'controller' and made sure he was comfortable with the situation. Weather was pretty good and improving (broken clouds but basically a VFR approach underneath). All worked fine during descent and approach. First officer was doing an excellent job-very smooth. We configured early; and I asked approach if we could join ILS at 5;000 ft to reduce power changes down the glideslope (instead of the normal step downs) and they cleared us to do so. From about 4;000 ft down; the pressurization began full scale swings both up and down. These were completely uncontrollable with the pressurization wheel; even though the aircraft was configured and power was stable. The procedure was to depressurize at 2;000 ft above ground and this was accomplished. Even depressurized with a constant 500 FPM aircraft descent; the abnormal fluctuations continued. [We] landed uneventfully. I checked with the cabin; and though the pressure changes were somewhat noticeable; they did not feel it warranted a PA to the passengers. We never did find out exactly why the fluctuations occurred with steady power on the aircraft; but this system definitely did not operate 'as advertised' for the last several thousand feet.

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Original NASA ASRS Text

Title: MD83 Captain experiences loss of both automatic pressurization controllers during cruise. Manual control is used to continue to destination but during approach with a steady power setting pressure fluctuations are noted and uncontrolable.

Narrative: We lost both pressure controllers at cruise. [We] switched to manual pressurization per QRH. No emergency was declared due to manual pressurization performing adequately. Briefed First Officer that he would be the 'controller' and made sure he was comfortable with the situation. Weather was pretty good and improving (broken clouds but basically a VFR approach underneath). All worked fine during descent and approach. First Officer was doing an excellent job-very smooth. We configured early; and I asked Approach if we could join ILS at 5;000 FT to reduce power changes down the glideslope (instead of the normal step downs) and they cleared us to do so. From about 4;000 FT down; the pressurization began full scale swings both up and down. These were completely uncontrollable with the pressurization wheel; even though the aircraft was configured and power was stable. The procedure was to depressurize at 2;000 FT above ground and this was accomplished. Even depressurized with a constant 500 FPM aircraft descent; the abnormal fluctuations continued. [We] landed uneventfully. I checked with the cabin; and though the pressure changes were somewhat noticeable; they did not feel it warranted a PA to the passengers. We never did find out exactly why the fluctuations occurred with steady power on the aircraft; but this system definitely did not operate 'as advertised' for the last several thousand feet.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.